1. Technical Field
The invention relates to optical gyros and more particularly to a method and apparatus for compensating for residual birefringence in interferometric fiber-optic gyros.
2. Background Art
Interferometric fiber-optic gyros used for sensing rate of rotation in aircraft and missile navigation applications are well known. One prior art passive ring, fiber-optic gyro arrangement, which senses rate of rotation about various axes, is described in detail in U.S. Pat. No. 4,828,389 to Gubbins et al., issued May 9, 1989. Such gyros typically include an optical signal source, a beam splitter, a phase modulator and a closed optical fiber ring. The beam splitter divides the light from the source into two beams of equal intensity which travel around the fiber-optic ring, one in the clockwise direction and the other in the counterclockwise direction resulting in two interfering, counter-propagating waves. As the beams emerge from the fiber-optic ring, they are recombined at the beam splitter and routed to an optical detector. The physical length of the clockwise path is identical to that of the counterclockwise path, and absent certain disturbances, the two beams are in phase when they are recombined, and maximum-intensity light is detected at the photodetector.
There are two types of disturbances that can give rise to phase shifts in the light waves, namely, those resulting in reciprocal phase shifts and those resulting in nonreciprocal phase shifts. A reciprocal phase shift occurs when the two light waves are affected by a disturbance in the same manner, and a nonreciprocal phase shift occurs when the disturbance affects one of the counter-propagating waves in a different fashion than the other. It is known, for example, that a nonreciprocal phase shift results from the rotation of the closed optical path, causing the counter-propagating light waves to require different amounts of time to complete a transit of the closed path. This phase shift is known as the Sagnac effect phase shift and provides an indication of angular displacement of the gyro. The use and measurement of the Sagnac effect in optical rate sensors is well known and described, for example, in the text entitled "Fiber-Optic Rotation Sensors and Related Technologies," Springer-Verlag 1982.
The intensity of light resulting from recombined, interfering, counter-propagating light waves may be expressed in terms of the trigonometric cosine waveform. When the phase difference between the counter-propagating waves is close to zero, the cosine function varies only slightly with changes in phase difference, and measurement of the phase shift is made difficult. The measurement of phase shift may be improved by introducing a nonreciprocal phase bias such as .pi./2 radians. The induced phase shift causes the intensity signal to be shifted to a more linear portion of the cosine signal curve in order to provide more accurate measurements of the Sagnac effect phase shift.
Because the light beam produced by the source is not always well polarized, two modes of polarization tend to be produced at the source and both can be propagated in the fiber-optic ring as counter-propagating waves. A difficulty is that the two modes of polarization do not propagate with the same velocity nor do they propagate with perfect independence, and coupling between the modes tends to occur. As a result, the detection of the Sagnac effect phase shift is made more difficult. It is well known to add a polarizer filter in the light path in order to enforce polarization in one direction. This technique works in principle if the light exiting the polarizer is highly polarized and the other optical components of the system do not repolarize the light beam. In practice, however, there exist various anisotropic influences in the optical ring and other optical devices, including polarizers, causing birefringence and modifying the state of polarization of the light. As a result, degenerate orthogonal modes of polarization tend to be propagated in the gyro fiber-optic ring, leading to inaccuracies at the detector and an output signal with excessive bias error. This general problem is well understood by those skilled in the art and referenced, for example, by Rashleigh and Stolen in "Preservation of Polarization in Single-Mode Fibers," Fiber-Optic Technology, pp. 155-161 May 1983.
Bias error resulting from birefringence of an optical device may be defined in terms of the amplitude extinction ratio (e) for that device and the gyro Sagnac scale factor K.sub.s as follows: ##EQU1## A typical value for K.sub.s is 5.mu. rad/degree/hour. Thus, for an acceptable bias error of 0.5 degrees/hour, the value of extinction ratio e has to be 3.5.times.10.sup.-6. Extinction ratios of this magnitude are generally considered to be unattainable or at least well beyond the current measurement capability. As a point of reference, the above-noted value corresponds to an extinction ratio expressed in terms of decibels of power is equivalent to -109 dB, which is not measurable with known measuring techniques. Currently available optical polarizers have extinction ratios in the -60 to -65 dB range.
The above-described birefringence problem can be alleviated to some extent by using a special Polarization Preserving Single-Mode Optical Fiber (PPSMOF). For a typical fiber ring having a length of 500 meters and using a good quality PPSMOF, it can be shown that a polarizer having an extinction ratio of -76 dB is needed in order to compensate for birefringence in the optical gyro with a bias error of 0.5 degrees per hour. The fabrication of a polarizer with such a low extinction ratio is difficult and costly to fabricate and 500 meters of good quality PPSMOF is also difficult to obtain and costly.
To improve the efficiency of the polarizer, it is well known to insert a depolarizer between the optical source and the polarizing filter. The effect of the depolarizer is to shift one of the degenerate polarization modes of the source signal in time, so that it is incoherent with respect to the other polarization mode. If the light beam is split into two independent beams traversing the gyro optical ring and the two independent beams are recombined, theoretically, there should be no measurable interference since the interference peak of the undesired polarization state is shifted beyond the coherence length of the desired mode. The use of a depolarizer, such as the well-known fiber Lyot depolarizer, tends to further reduce the extinction ratio requirement of the polarizer.
Typically, optical gyro systems include additional optical elements such as beam splitters/recombiners and optical couplers, all of which are inherently birefringent. Consequently, optical signals propagated in the optical circuits will be repolarized to some extent by each of these devices. The repolarization due to the birefringence of these circuit devices affects the bias error of the gyro.
It is an object of this invention to provide an interferometric fiber-optic gyro having low bias error and in which the effects of birefringence are minimized.